1. The field of the Invention
This invention relates to improvements in a pressure (operated) switch adapted to open and close in response to a pressure within a pressure circuit including a fluid pump used, for example, in an automotive power steering system.
2. The description of the Prior Art
A variety of hydraulic (oil) pumps have been proposed and put into practical use for automotive vehicle power steering systems. Typical ones of them are of rotary volume type oil pumps such as vane-type oil pumps. Such pumps are usually driven by an engine and arranged to rise in rotational speed to increase the discharge amount of oil as the engine speed increases. The pump forms part of a hydraulic circuit for generating a steering assisting power in the power steering system. More specifically, in the hydraulic circuit, the flow amount of oil from the pump is controlled by a flow control valve and then supplied to a power cylinder to assist steering.
The amount of oil to be supplied to the power cylinder corresponds to the magnitude of a steering assist force generated depending on the hydraulic fluid amount. In other words, the steering assist force is relatively increased when a steering wheel is turned during parking or during a low speed vehicle cruising, whereas it is relatively decreased when the steering wheel is turned during a high speed vehicle cruising. This is realized by arranging the flow control valve such that the flow amount of oil to be supplied to the power cylinder is decreased during the high speed vehicle cruising in which the pump rotational speed is relatively high. In this regard, the flow control valve has a so-called flow-down function to decrease the supply amount of oil when the rotational speed of the pump reaches a predetermined level, thereby supplying a required amount of the hydraulic pressure to the power cylinder in accordance with a required magnitude of the steering assist power without being affected by a rotational speed change of the pump or a pressure variation at a part of the hydraulic circuit leading to the power cylinder.
Additionally, the hydraulic circuit of the power steering system is provided with a pressure (operated) switch which is switched ON when the pressure within the hydraulic circuit exceeds a predetermined level upon an increase of load applied to the pump under an operation of the power steering. Such an increase of the pressure may cause an engine stall. When the pressure switch is switched ON, a device is operated to increase the engine speed of the engine mounted on the automotive vehicle.
An example of such a conventional pressure switch will be discussed with reference to FIGS. 4 and 5. The conventional pressure switch includes a switch housing 30 threadedly fitted in a bore of a support base. A stationary terminal 31 is fixedly supported by the switch housing 30. A movable terminal 32 is movably disposed in low pressure chamber to be contactable with the stationary terminal 31. The movable terminal 32 is biased by a spring 33 to separate from the stationary terminal 31. The movable terminal 32 is fixedly provided with a pressure receiving piston or pin 34 to which an oil pressure in a part 40 of a hydraulic circuit acts, so that the oil pressure is transmitted through the piston 34 to the movable terminal 32. The movable terminal 32 is kept separate from the stationary terminal 31 under a normal condition in which the oil pressure in the hydraulic circuit is lower than a predetermined level.
Thus, the part 40 serves as a high pressure chamber whereas a chamber in which the movable terminal 32 is disposed serves as a low pressure chamber. Accordingly, the movable terminal 32 is moved in response to the pressure difference between the high and low pressure chambers, and is brought into contact with the stationary terminal 31 when the oil pressure in the high pressure chamber 40 reaches the predetermined level. As a result, the pressure switch is switched ON or closed to operate a required device.
However, drawbacks have been encountered in the above discussed conventional pressure switch as set forth below. An operational oil pressure by which switching of the pressure switch is made is unavoidably determined only in response to the pressure difference between the high and low pressure chambers. In other words, there is no difference in the operational oil pressure between a time the pressure switch is switched ON and a time it is switched OFF, thus never providing a so-called hysteresis effect by which a delay is made when the pressure switch changes from its ON to OFF operations.
Accordingly, in case that such a pressure switch is used in combination with a power steering system in such a manner that the above-mentioned device is operated to increase the engine speed when the pressure switch is switched ON, the hysteresis effect cannot be obtained between the operational oil pressures of switching ON and OFF the pressure switch. Accordingly, if the oil pressure to be applied to the movable terminal frequently changes near the operational oil pressures, the pressure switch repeats its ON and OFF operations thereby causing chattering. Additionally, this frequently repeatedly puts the engine speed increasing device in its operative or inoperative state, thereby causing a large engine speed fluctuation.